A liquid crystal display device is now being widely used in a TV, a monitor for PCs, a monitor for PDAs, and the like, because it is a display device with light weight, slim profile, and low electrical power consumption. According, to such a liquid crystal display device, a transmittance of light that passes through a liquid crystal layer is controlled by a tilt angle of a liquid crystal molecule, the tilt angle depending on a voltage applied between a pair of substrates (to the liquid crystal layer). So in the liquid crystal display device, the transmittance depends on a viewing angle. Accordingly, in conventional liquid crystal display devices, display defects are observed in some viewing directions. For example, a contrast ratio is reduced or gray-scale inversion occurs when an image is displayed in an intermediate gradation level. Accordingly, such conventional liquid crystal display devices generally have room for improvement in viewing angle characteristics.
An alignment division technology in which each pixel is divided into two or more regions different in tilt direction of liquid crystal molecules is now being researched and developed. According to this technology, the viewing angle characteristics can be improved by tilting the liquid crystal molecules in each pixel in different directions by applying a voltage to the liquid crystal layer. The respective regions which differ in the alignment direction of the liquid crystal molecules are also called domain. The alignment division is also called multi-domain.
As the multi-domain liquid crystal display devices, examples of horizontal alignment liquid crystal display devices include: a multi-domain TN (twist nematic) liquid crystal display device; a multi-domain ECB (electrically controlled birefringence) liquid crystal display device; and a multi-domain OCB (optically compensated birefringence) liquid crystal display device. In addition, examples of vertical alignment liquid crystal display devices include: a MVA (multi-domain vertical alignment) liquid crystal display device; a PVA (patterned vertical alignment) liquid crystal display device; and a multi-domain VAECB (vertical alignment electrically controlled birefringence) liquid crystal display device. These various liquid crystal display devices are now being further developed in order to show improved viewing angle characteristics.
A rubbing method, a photo-alignment method, and the like, are mentioned as a method for the alignment division. In the rubbing method, for example, an alignment film is rubbed, with a rubbing region and a non-rubbing region being separated from each other by a resist pattern. According to such a rubbing method, the alignment film surface is provided with the alignment treatment by being rubbed with a cloth wound on a roller. In this method, dusts such as a fiber of the cloth and rubbed scrapes are generated, and further, static electricity generated due to the fiber of the cloth might cause defects such as malfunction, a change in characteristics, and deterioration, of switching elements. In such a point, the rubbing method still has room for improvement.
In the photo-alignment method, a photo-alignment film is used as a material for the alignment film, and the photo-alignment film is exposed to light such as ultraviolet light, and thereby the alignment film is provided with an alignment regulating force and/or an alignment regulating direction of the alignment film is changed. Thus, in the photo-alignment method, the alignment film can be subjected to the alignment treatment in a contact-less manner, and so, soils, dusts, and the like that are generated by the alignment treatment can be reduced. In addition, in the photo-alignment method, by exposing the alignment film through a mask, desired regions in the alignment film plane can be photo-irradiated under different conditions. As a result, domains having a desirable design can be easily formed.
The following method is mentioned if one pixel is divided into two domains by the common photo-alignment method. Using a photomask where transmissive parts like slits each having a width about half of a pixel pitch are formed within a light-shielding region, a half region of the pixel is subjected to the 1st exposure, and the photomask is shifted by a half pixel pitch and then the rest region of the pixel is subjected to the 2nd exposure under conditions different from those in the 1st exposure. As a result of the 1st and 2nd exposure, each pixel can be easily divided into two or more domains. In addition, for example, Patent Document 1 discloses the following liquid crystal alignment film as a technology of providing a liquid crystal vertical alignment film that can provide multi-domain alignment without being rubbed, and a technology of providing a liquid crystal display device including such a film. The liquid crystal alignment film is composed of a thin film like a monomolecular film, and a molecule constituting the thin film has a fluorinated carbon group and a photosensitive group and is bonded and fixed to a substrate surface at an end part of the molecule constituting the thin film. Also the molecules constituting the thin film are aligned in a plurality of directions in a pattern per region of the thin film, and the molecules constituting the thin film are polymerized or cross-linked with themselves via the photosensitive groups. In addition, for example, Patent Document 2 discloses the following liquid crystal display device as a technology of stably aligning liquid crystals by a photo-alignment method and thereby achieving excellent display. The liquid crystal display device includes an alignment film composed of a mixture of polyamic acid including a diamine component and polyimide including a diamine component, the two diamine components being different, and this alignment film is irradiated with UV light, thereby being subjected to alignment treatment.
In addition, an increase in size of the liquid crystal display device is now rapidly proceeding. Liquid crystal TVs in 40 to 60-inch model are being rapidly developed, although plasma TVs have conventionally accounted for the greatest share of devices in such sizes. However, it is very difficult to provide a large liquid crystal display device such as a device in 60-inch model with alignment division by the above-mentioned conventional photo-alignment methods. The reason is given below. There is no exposure apparatus that can complete exposure for a substrate in 60-inch model through one exposure and can be placed at a plant, and so it is impossible to complete exposure for the entire substrate surface in 60-inch model through one exposure. Accordingly, the exposure for the substrate needs to be completed through several exposures, when a large liquid crystal display device is subjected to the alignment division by the photo-alignment method. In addition, when a relatively small liquid crystal display device in 20-inch model is subjected to the alignment division by the photo-alignment method, exposure for the substrate would be completed through several exposures if the exposure apparatus needs to be reduced in size as much as possible. However, in the liquid crystal display device including the substrate the exposure for which is completed through several exposures, a joint line between exposure regions might be clearly observed on a display screen, possibly resulting in inferior product. Accordingly, if the liquid crystal display device is subjected to the alignment division by completing the exposure for the substrate through several exposures, there is still room for improvement in display qualities and yield.
For example, Patent Document 3 discloses the following production method of a liquid crystal display device, as an exposure technology of pattern-forming a switching element, an electrode, and the like. The production method includes steps of: forming a resist film on a substrate on which a plurality of pixels are to be arranged; transferring a first mask pattern to a first region of the resist film by exposing the first region, which is a part of the resist film, through a first mask; transferring a second mask pattern to a second region of the resist film by exposing the second region, which partially overlaps with and is adjacent to the first region of the resist film, through a second mask, wherein the step of transferring the first mask pattern includes a step transferring the first mask pattern portion to one part of a pixel being in a position at an overlap exposure area, in which the first and the second areas are laid over each other, and the step of transferring the second mask pattern includes a step of transferring the second mask pattern to another part of the pixel.
[Patent Document 1]
Japanese Kokai Publication No. 2001-281669
[Patent Document 2]
Japanese Kokai Publication No. 2003-43492
[Patent Document 3]
Japanese Kokai Publication No. 2000-66235